216 American Fisheries Society 



popularly reported in this country arc caused by the same 

 or related parasites. 



Among the parasitic worms three types occur in fish : 

 The Trematoda or flukes, the Cestoda or tapeworms, and 

 the Nematoda, roundworms or threadworms. A few of the 

 flukes are found living on the skin of fish as external para- 

 sites, but these are confined to marine species and deserve 

 only passing mention here. All the rest of the forms men- 

 tioned which are parasitic in fish occur within the body and 

 are regarded as internal parasites, even though some of them 

 on the mouth or on the gills have practically the same condi- 

 tions of existence as are found on the external surface and 

 are very similar in structure to the species of external para- 

 sites that live on the skin of marine fish. 



A few of these internal parasites live in the body cavity 

 and come to notice when the fish are cleaned, a few others 

 occur embedded in muscle tissue or at other points and are 

 sufficiently conspicuous to attract attention at that time or 

 later, but the large majority of such forms inhabit the ali- 

 mentary canal, liver, swimming bladder, or other visceral 

 organs. They are removed in toto with the viscera and 

 only rarely are recognized as present. They are neverthe- 

 less the most important forms both from the hygienic and 

 from the economic standpoint. 



Where only a few internal parasites are present their 

 influence on the fish is relatively unimportant except in so 

 far as they form the basis for an infection which under 

 favorable circumstances may increase to far greater pro- 

 portions. The breaking out of parasitic epidemics is well 

 known and is justly regarded with apprehension, for both 

 among wild fish and among such as are held in control, such 

 epidemics have appeared suddenly and in a brief period have 

 swept away the results of years of labor on the part of the 

 fish culturist. But even where no epidemic breaks out there 

 is a distinct loss, since the influence of parasites reduces the 

 reproductive power of the fish and also its power to grow 

 and to lay on flesh. Now these are precisely the factors in 



